Downloading the app:

If you're on PC, you can scan QR code with your Android device.
On device you can use "Download APK" button.

Installing the app:

Before you can actually install the APK file, you'll need to tell your device that it can install any apps outside of the Play Store without throwing a fit.

You can find this switch under Settings » Security » Unknown Sources

Once this is enabled, you're clear to fire up your favorite file browser, locate the APK file, and open it.
Android should take over, identifying that the APK file is an app, and ask you whether you'd like to go ahead and install it, all permissions accounted for.
After a couple seconds, app should be installed and ready to go!